1590s, "be in harmony, agree, be in accordance," from adverbial phrase atonen (c. 1300) "in accord," literally "at one," a contraction of at and one. It retains the older pronunciation of one. Meaning "make up (for errors or deficiencies)" is from 1660s; that of "make reparations" is from 1680s.

Atone. To bring at one, to reconcile, and thence to suffer the pains of whatever sacrifice is necessary to bring about a reconciliation. [Hensleigh Wedgwood, "A Dictionary of English Etymology," 1859]

The phrase perhaps is modeled on Latin adunare "unite," from ad "to, at" (see ad-) + unum "one." Related: Atoned; atoning.

"being but a single unit or individual; being a single person, thing, etc. of the class mentioned;" as a pronoun, "a single person or thing, an individual, somebody;" as a noun, "the first or lowest of the cardinal numerals; single in kind, the same; the first whole number, consisting of a single unit; unity; the symbol representing one or unity;" c. 1200, from Old English an (adjective, pronoun, noun) "one," from Proto-Germanic *ainaz (source also of Old Norse einn, Danish een, Old Frisian an, Dutch een, German ein, Gothic ains), from PIE root *oi-no- "one, unique."

Originally pronounced as it still is in only, atone, alone, and in dialectal good 'un, young 'un, etc.; the now-standard pronunciation "wun" began c. 14c. in southwest and west England (Tyndale, a Gloucester man, spells it won in his Bible translation), and it began to be general 18c. Its use as indefinite pronoun was influenced by unrelated French on and Latin homo.

Before the name of a person, indicating "hitherto unknown" or not known to the speaker.

One and only "sweetheart" is from 1906. Slang one-arm bandit for a type of slot machine is recorded by 1938. One-night stand is 1880 in performance sense; 1963 in sexual sense. One of the boys "ordinary amiable fellow" is from 1893. One-track mind "mind capable of only one line of thought or action" is by 1915. Drinking expression one for the road is from 1950 (as a song title). One-man band is by 1909 in a literal sense, 1914 figurative. One of those things "unpredictable occurrence" is from 1934 (Cole Porter's song is from 1935).

The conscience clause is one of the weaknesses of the Bill. It is one of those things which tend to create the bitterness. The conscience clause is one of those things which are inseparable from a Bill like this. It is one of those things which divides the sheep from the goats—members can pick them out for themselves—in the playground, in the school. ["Religious Exercises in School Bills," New Zealand Parliamentary Debates, Aug. 13, 1926]

c. 1200, "to repair" (clothes, a tool, a building), "remove defects" (from something broken, defaced, deranged, or worn), from a shortened form of Old French amender "correct, set right, make better, improve" (see amend). Meaning "to put right, atone for (faults and errors), amend (one's life), repent" is from c. 1300. Intransitive sense of "to grow better, improve" is from late 14c. Related: Mended; mending.

"apply medicinal or sacramental ointment to," Middle English salven, from Old English sealfian "anoint (a wound) with salve," from Proto-Germanic *salbojanan (source also of Dutch zalven, Old Frisian salva, German salben, Gothic salbon "to anoint"), from the root of salve (n.).

Figurative use is by late 12c. in reference to sin or vice; the non-religious sense of "to help, remedy, atone for" is by 1570s. Related: Salved; salving.